This is a national stage of PCT application No. PCT/FR00/02243, filed on Aug. 3, 2000. Priority is claimed on that application, and on patent application No. 99/10418 filed in France on Aug. 9, 1999.
The present invention relates in general to the field of bottom-to-surface type connections of the type including a vertical underwater pipe referred to as an up column or riser, connecting the sea bed to a floating support installed on the surface.
Once the depth of water becomes large, production fields and in particular oil fields are generally operated from floating supports. When operating an off-shore hydrocarbon deposit at great depth, it is preferable for economic reasons that the floating support structure be capable of performing all of the operations concerned with drilling wells, putting them into production, and reconditioning them. It is also preferable for the floating support to be capable of drilling a plurality of wells in succession over the same site and subsequently to be suitable for performing production and reconditioning operations for each of said wells. In general, the floating support has anchor means to keep it in position in spite of the effects of currents, winds, and swell. It generally also includes oil storage and processing means together with means for off-loading to off-loading tankers. Such tankers arrive at regular intervals to off-load production. Such floating supports, which are generally of the barge type, are commonly referred to by the initials xe2x80x9cFPSOxe2x80x9d for floating production storage off-loading. Numerous variants have been developed such as SPARs (which are long vertical floating cigar-shaped objects held in position by catenary anchoring) or indeed TLPs (tension leg platforms) which are platforms whose xe2x80x9clegsxe2x80x9d are anchor lines under tension, which lines generally extend vertically.
Wellheads are often distributed over an entire field and production pipes, together with water injection lines and inspection and control cables, are all placed on the sea bed to extend towards a fixed location vertically below the position of the floating support on the surface.
Some wells are thus situated vertically beneath the floating support and the insides of such wells can then be accessed directly from the surface. Under such circumstances, a wellhead fitted with its xe2x80x9cChristmas treexe2x80x9d can be installed on the surface on board the floating support. It is then possible to use a derrick installed on said floating support to perform all of the drilling, production, and maintenance operations required on the well throughout its lifetime. This is referred to as a xe2x80x9cdryxe2x80x9d wellhead.
With SPARs and TLPs, dry wellheads are brought together in a limited zone of the platform over which the derrick is displaceable to take up a position vertically over each of the wells so as to perform drilling operations or maintenance operations on a well throughout the lifetime of the field.
Since a drilling derrick is a tall structure, possibly 60 meters (m) tall and possessing a hoisting capacity that can exceed 500 tonnes (t), the means required for moving it from one well to another and for keeping it in position during operations on any particular well give rise to structures that are complex and expensive.
In order to maintain the riser fitted with its dry wellhead in a substantially vertical position, it is appropriate to exert upward traction which can be applied either by a cable tensioning system using winches or hydraulic actuators installed on the floating support, or else by means of floats distributed along the riser and installed at various depths, or indeed by a combination of those two techniques.
French patent FR 2 754 011 describes a barge and a guidance system for a riser, in which the riser is fitted with floats.
SPARs and TLPs are likewise fitted with a multiplicity of risers supported by floats constituting cans that surround a riser coaxially, and that are held in position by guidance systems.
In FPSOs, risers reach the surface in a central cavity of the floating support that is referred to as a xe2x80x9cwellbayxe2x80x9d. The cavity passes right through the hull vertically over a height of about 30 meters, drawing water to a depth of about 20 meters. It is generally installed on the axis of the floating support, at equal distances from its ends since this is the zone in which the amplitudes of movements and of accelerations are the smallest when the vessel is subjected to the phenomena of roll, pitch, and yaw.
The depth of water over some oil fields exceeds 1500 m, and can be as much as 2000 m to 3000 m, so the weight of risers for such depths requires vertical forces to be deployed to keep them in position that can be as great as or more than several hundreds of tonnes. Buoyancy elements of the xe2x80x9ccanxe2x80x9d type are used which are installed at various levels along risers connecting the surface to ultragreat depths (1000 m-3000 m).
The floats concerned are of large dimensions and in particular they have a diameter in excess of 5 m, and a length of 10 m to 20 m, with each float delivering buoyancy that can be as much as 100 tonnes.
The float and the riser are subjected to the effects of swell, and of current, but since they are connected to the FPSO on the surface, they are also indirectly subjected to the effects of wind. This gives rise to lateral and vertical movements of the assembly comprising the riser, the floats, and the barge, which movements can be large, as much as several meters, particularly in a zone that is subject to swell.
To ensure that risers do not interfere with one another and do not interfere with the hull of the floating support, said risers are spaced apart from one another by several meters and also by several meters from the walls of the wellbay, which means that such wellbays can often be as much as 80 m long and 20 m wide on FPSO barges that are themselves as much as 350 m and 80 m wide, and rising by as much as 35 m above the water line. Such barges have a deadweight that can be as much as or greater than 500,000 tonnes deadweight.
These riser movements give rise to large differential forces between a riser and the guidance systems secured to the floating support.
The amplitude of the displacements and the very high level of the forces in the risers make it necessary to design guidance systems capable of withstanding not only extreme conditions, but also phenomena of fatigue and wear of the kind that can accumulate over the lifetime of such an installation, which can exceed 25 years. The present invention relates to transferring risers within the preferably central cavity, from a production position to a position in which drilling operations and heavy maintenance operations can be performed on a well, i.e. using a derrick that is fixed relative to the floating support.
Well drilling is thus performed on the main axis of the derrick through a xe2x80x9cdrillingxe2x80x9d riser whose function is to guide the drill strings and to contain the mud returned from a well that is being drilled. Such a drilling riser is made up from unit lengths that can be as much as 50 m long, with the entire assembly being lowered step by step as the said riser is assembled. At the end of drilling, the portion of the riser corresponding to the depth of water is disconnected from the well at the sea bed, and is then taken to a parking position after its length has been shortened by removing one or two of the unit lengths. By proceeding in this way, the drilling riser remains suspended with its bottom end situated at 50 m to 100 m from the sea bed.
The production riser can then be assembled step by step in the same manner until it reaches the entrance to the well. Floats are installed on the top portion as it is put into place, and finally the bottom of the production riser is connected to the well. The well is then fitted with various items of production tubing and the xe2x80x9cChristmas treexe2x80x9d of the dry wellhead is put into place.
The assembly is then transferred to a production position in which it will remain throughout the lifetime of the field, except when certain maintenance operations are performed that require said riser to be returned to the main axis of the derrick in order to perform heavy maintenance operations.
Such guidance systems generally comprise tensioning by means of cables which make transferring a riser from one position to another within a wellbay lengthy and difficult to perform, particularly with reference to displacement from the production position to the position where it lies on the axis of the derrick.
In FR 2 747 728, a floating support is described that has a rectangular or circular bay enabling a group of hydrocarbon wells to be drilled and/or operated at great depth without it being necessary to displace the derrick relative to the deck of the floating support in order to operate on a particular well selected from the group of wells, and without it being necessary to use a traveling crane to hold and displace an individual riser selected from the group of wells so as to make it possible to operate on the selected well.
In that patent FR 2 747 728, the risers are fixed at intervals to one another to a moving table that can be moved relative to the deck of the floating structure along a path such that the top ends of the respective risers can be brought successively into vertical alignment with the derrick. The bay can be in the form of an elongate rectangle and the moving table is then rectangular, extending in the longitudinal direction of the rectangular bay and movable in translation in its longitudinal direction. When the moving table is moved in translation along its longitudinal axis, all of the risers are moved simultaneously. The bay could also be circular in section with the moving table being likewise circular, having a diameter corresponding to that of the circular bay and disposed concentrically therewith, being mounted to rotate within the circular bay about the vertical axis thereof. In that case also, rotation of the circular table about its axis causes all of the risers that are attached thereto to move simultaneously.
A drawback of that system is that displacement of the moving rectangular table or rotation of the moving circular table causes all of the risers to be deflected relative to the vertical. This causes tension and/or twisting forces to be applied to the risers giving rise to a force component which, by reaction on the floating structure, gives rise to considerable levels of force between the floating support and said table, and when the table is circular, giving rise to couples tending to cause said structure to turn relative to the floating support about the axis of the circular table. Turning the circular table about its axis likewise subjects each riser to twisting forces.
FR 2 747 728 also describes a platform in which the risers are grouped together in a rectangular zone in a matrix configuration which, in plan view, comprises four rows and six columns. The derrick which supports the devices enabling well drilling, production, and reconditioning operations to be performed is mounted to move over the deck of the platform by means of a slideway system, so that the derrick can be brought selectively into vertical alignment with the top end of any one of the risers. Given that the surface wellheads situated at the top ends of the risers must have a center-to-center spacing of several meters, the derrick needs to be displaceable over a rectangular zone of relatively vast extent and the support structure connecting it to the floating support must be capable of being placed astride two longitudinal rows of wellheads so that the axis of the derrick can be placed in line with the axis of each wellhead on which it might be desired to take action. Since such a support structure should be capable of moving longitudinally and transversely relative to the axis of the floating support, it needs to be of considerable size and complexity, and it is therefore very expensive. Furthermore, when the derrick is acting on a side row, the weight of the derrick and the vertical load supported thereby can reach or exceed 3500 tonnes, so it is necessary to have large means for balancing the load on the floating support, e.g. in the form of ballast.
FR 2 747 728 also discloses a platform having a plurality of risers disposed at the internal periphery of a rectangular wellbay. A traveling crane that moves over the bay is designed to take charge of any of the risers so as to bring a selected riser from its respective support at the periphery of the bay to the center of said bay in vertical alignment with a derrick so as to enable well drilling, production, or reconditioning operations to be performed on the selected riser. Such a traveling crane must include a tensioning system capable of maintaining the riser in tension while it is being transferred from its production position to its position on the axis of the derrick where well drilling or reconditioning operations are performed. The vertical tensioning forces for supporting a deep water riser can reach or exceed 300 tonnes or even 500 tonnes or more, so the structure of the traveling crane needs to be strong since it must extend all the way across the wellbay and it must be capable of traveling beneath the structure of the derrick. The base of said derrick then needs to be enlarged considerably and reinforced so as to allow the traveling crane to pass freely.
The object of the present invention is to provide a novel type of floating support in which transferring a riser from one position to another within the wellbay is simple to perform and can be done in complete safety.
More particularly, an object of the present invention is to provide a novel type of floating support in which risers can be transferred from a production position to a drilling position within the wellbay by displacing the risers individually but without requiring an expensive infrastructure such as a traveling crane.
To do this, the present invention provides a floating support which has a preferably central cavity containing a plurality of compartments at the ends of which said risers are in their production positions, said compartments communicating with a central channel at the end of which a drilling derrick is situated, said compartments being disposed transversely relative to said central channel.
Advantageously, said compartments are essentially rectangular in longitudinal shape and they are disposed parallel to one another.
Each compartment thus contains a single riser at most, installed at the closed end of said compartment and capable of being displaced individually amongst the compartments and the central channel.
The buoyancy of the risers is preferably provided essentially or exclusively by floats without adding any additional tensioning system, in particular any system making use of cables and winches or hydraulic actuators installed on the floating support. This considerably facilitates displacement of the risers.
In a particular embodiment, said compartments are longitudinally rectilinear in shape and they are disposed parallel to one another, perpendicularly or at an angle relative to a central one of said rectilinear channels. They are preferably spaced apart from one another in regular manner.
Advantageously, said compartments and said central channel comprise a junction floor between said riser and said floating support on each rim, said junction floor defining a continuous channel of substantially constant width corresponding to a distance that is sufficient for installing a riser between its two rims and for enabling it to be displaced with the help of transfer means situated on said junction floor.
In a particular embodiment, said junction floor is situated at an intermediate level, in particular halfway, between the deck of the floating support and water level.
Preferably, the floating support of the present invention has means for transferring said risers between their production positions at the ends of the compartments, and the position of the derrick, said transfer means enabling one of said risers to be moved along said junction floor by co-operating with said risers at said junction floor.
In an embodiment, when the riser is fitted with apparatus for holding and guiding the riser in its movements relative to the floating support, said holding and guiding apparatus for the riser co-operates in particular with said riser and with said floating support at said junction floor, said transfer means co-operating with apparatus for holding and guiding the riser, said apparatus providing the junction between said riser and said floating support.
In a variant embodiment, said transfer means are stationary and secured to said floating support.
In particular, said transfer means comprise a set of winches and cables connecting said winches to said riser.
Advantageously, said transfer means comprise a carriage that is movable along said compartments and along said channel.
Also advantageously, said junction floor is fitted with guide rails enabling said transfer means or said riser to be guided, in particular if said transfer means are stationary, along or inside said compartments and said central channel, in particular where said compartments intersect said central channel.
In an embodiment, the riser is fitted with a support which provides a junction with the junction floor comprising guide elements, preferably disposed beneath said support and secured thereto, enabling said riser to be guided inside said compartments and said central channel, and where appropriate, inside said guide rails.
If the riser is fitted with holding and guidance apparatus acting relative to the floating support, it is the support providing the junction between the holding and guidance apparatus and the junction floor which has said guidance elements.
The floating support of the present invention can also include a compartment extending transversely to the central channel on the axis of the derrick, said compartments corresponding to a parking position for a riser.
In an advantageous embodiment, said risers are fitted with can-shaped floats surrounding each riser in coaxial manner about the top portion of the riser, the buoyancy of the risers being provided essentially by said floats without adding any additional tensioning system installed on the floating support.
The present invention also provides a method of transferring a riser on a floating support from a production position to a position within a drilling derrick, in which method said riser is displaced in said cavity of a floating support of the invention, as explained in the detailed description.
In an advantageous implementation, said riser is displaced by displacing said riser holding and guidance apparatus relative to the floating support, which apparatus is secured to said riser.
The apparatus for holding and guiding the riser relative to the floating support enables relative movements to be made between the floating support and the riser so as to support common loading forces corresponding to loads that can be as great as 10 tonnes, and occasional extreme forces corresponding to loads that can be as great as 100 tonnes, or even 200 tonnes or more.
In any event, the apparatus for holding and guiding the riser relative to the floating support must be capable of allowing longitudinal displacement in a substantially vertical direction of 5 meters or even more. Furthermore, for operations performed on the axis of the derrick, the riser must remain substantially in line with the axis of the derrick.
Advantageously, the apparatus for holding and guiding the riser relative to the floating support includes joint means secured to said floating support and making the following possible:
a) said riser can turn about a horizontal axis perpendicular to the longitudinal axis of said riser within a cone having a half-angle at the apex that is less than or equal to 10xc2x0, said horizontal axis and said apex of the cone being situated substantially at the center of the riser and level with the midplane of the zone in which said joint means on the longitudinal axis are situated; and
b) said sliding of said riser along the longitudinal axis and said guidance of lateral displacements of said riser in a horizontal plane perpendicular to said longitudinal axis of the riser; and
said joint means comprise:
friction pads presenting bearing surfaces for bearing against said riser, said pads being mounted on a pad support enabling said pads to pivot about respective axes perpendicular to said longitudinal axis of said riser; and preferably
said pads co-operate with wheels so that said wheels bear against said riser and allow it to slide, and said riser bears against said pads only when said wheels are displaced under the effect of lateral displacements of said riser.
The term xe2x80x9clongitudinal axis of the riserxe2x80x9d is used to mean the vertical axis when the riser is in its rest position, i.e. when it is not subjected to movements associated with the sea moving.
The apparatus for holding and guiding the riser relative to the floating support is designed to support forces that vary in a horizontal plane. The wheels preferably serve to guide the riser while it is subjected to small everyday forces of about 10 tonnes, and the pads provide guidance when the riser is subjected to heavy loading under extreme conditions, and in particular forces of up to 100 tonnes.
The apparatus for holding and guiding the riser relative to the floating support can be put into place on the floating support to hold and guide one of said risers in its production position, or it can be connected to said means for transferring one of said risers from its production position to the axis of a drilling derrick. In both cases, said joint means are preferably disposed at the junction floor between said riser and said support, situated in the wall of the cavity of the floating support between the deck of the floating support and water level.
The holding and guidance apparatus can also be installed on said floating support to hold and guide one of said risers in its operating position under a derrick installed on said floating support, and more precisely on the axis of said derrick.